Method for operating at least one air separation apparatus and oxygen consumption unit

ABSTRACT

The present invention relates to a process for operating at least one air separation unit and an oxygen-rich gas consumer, comprising a carbon fuel combustion unit or a gasification unit, the oxygen-rich gas consumer being capable of generating electricity. The consumer is supplied with an oxygen-rich gas coming from the air separation unit or units.

The present invention relates to a process for operating at least one air separation unit and an oxygen-rich gas consumer, comprising a carbon fuel combustion unit or a gasification unit, the oxygen-rich gas consumer being capable of generating electricity. The consumer is supplied with an oxygen-rich gas coming from the air separation unit or units.

One of the CO₂ capture technologies for carbon fuel combustion units, for producing energy, called oxy-fuel combustion, will require very large amounts of oxygen (from 10 000 M tonnes per day to 20 000 M tonnes per day, depending on the site) produced by a series of air separation units associated with units for separating the waste gases from combustion units for producing CO₂ at the outlet of one or more combustion units before the transportation and sequestration thereof. These air separation units are very large consumers of electrical energy, thereby penalizing the feeding into the grid of the power produced by the consumer at the times when the energy cost is highest.

Switching systems are known that allow the power consumed by them during periods of peak demand to be limited (US-A-20080115531 or WO-A-09/071,833).

It is also conceivable to shut down and restart air separation units in order to save energy and thus switch to air mode for the consumer, not capturing the CO₂ except during these periods of relatively short duration, but the time for restarting is not necessarily compatible with several operations of this type each day.

Some combustion units are designed to operate in base mode, that is to say in continuous stable operation for the entire or almost entire year (high season, intermediate season and low season), and to do so generally close to their nominal consumption, so as to feed the electrical energy continuously into the grid, while other combustion units are designed to operate more erratically and to meet the requirements above a certain level of electrical energy consumption (high and intermediate seasons), while yet others are designed to respond solely to peak demands (for a few hundred, or even a little more than one thousand, hours per year, high season).

In the case of combustion units operating in a relatively erratic manner, the air separation units that deliver the oxygen for oxy-fuel combustion are designed to deliver, to their nominal level, all the requirements of the consumer when it is operating normally, and are obliged to vent to atmosphere or to shut down when the combustion unit stops for a few hours each day or for a few days per week when the demand is low, the overall consequence of which is a considerable loss of energy.

This is because venting the oxygen to atmosphere after it has been separated, even if it may be carried out at low pressure, represents an amount of lost energy of around 0.2 to 0.35 KWh/Nm³, depending on the process schemes used.

As for restarting an air separation unit after a shutdown of short duration, apart from its complexity, this will take in the region of one hour before obtaining the required purities and pressures, also corresponding to a not insignificant energy loss.

According to one aspect, the invention provides a process for operating a plant comprising at least two air separation units, a storage system and an oxygen-rich gas consumer comprising a carbon fuel combustion unit or a gasification unit, the consumer being capable of generating electricity at least in a first operating phase, in which process the plant operates in a plurality of operating phases:

-   -   a) in the first operating phase, during which the cost of the         electricity is above a first price threshold, the consumer         receives an amount of oxygen-rich gas above a first consumption         threshold, coming from at least one of the air separation units,         and the oxygen-rich gas consists partly of oxygen stored in the         storage system, which is supplied by the air separation units         during a second operating phase, and partly by oxygen produced         by distillation in at least one air separation unit during the         first operating phase; and     -   b) in the second operating phase, during which the cost of the         electricity is below a second price threshold, the second price         threshold being below the first price threshold, the consumer         consumes an amount of oxygen-rich gas below a second consumption         threshold, the second threshold being below the first threshold,         the amount possibly being zero, air is separated in at least         certain of the air separation units, and an oxygen-rich liquid         is sent from the separation unit or from the at least one         separation unit to the storage system; and     -   c) the number of separation units in operation during the first         operating phase is fewer than the number of units that are in         operation during the second operating phase.

According to other optional aspects:

-   -   the number of moles of oxygen in the oxygen-rich liquid stored         throughout the duration of the second operating phase is fewer         than the number of moles of oxygen sent as oxygen-rich gas to         the consumer during the first operating phase;     -   the oxygen-rich gas consumption during the first operating phase         is substantially constant;     -   the amount of air sent to at least one of the air separation         units, being the unit or units in operation during the first         operating phase, corresponds to an oxygen production at least         15% less, preferably at least 25% less or even at least 40% less         than the production of gaseous oxygen sent to the consumer;     -   the difference between the production of oxygen-rich gas         corresponding to the amount of air sent to the at least one         separation unit in operation during the first operating phase         and the consumption of oxygen-rich gas sent to the consumer         corresponds to at least part of the amount of liquid oxygen         stored during the second operating phase;     -   the number of separation units in operation during the first         operating phase is at least two fewer than the number of units         in operation during the second operating phase;     -   the number of air compressors supplying an air separation unit         and in operation during the first operating phase is preferably         at least two fewer than the number of air compressors supplying         an air separation unit in operation during the second operating         phase;     -   the amount of liquid oxygen sent from the air separation unit or         units to the storage system during the first operating phase         does not exceed 1%, preferably 2% or even 5% of the flow of air         sent to the air separation unit(s);     -   the amount of liquid oxygen sent from the storage system to the         air separation unit or units during the second operating phase         does not exceed 1%, preferably 2% or even 5% of the flow of air         sent to the air separation unit(s);     -   the amount of gaseous oxygen withdrawn from the air separation         units (1) during the second operating phase does not exceed 1%,         preferably 2% or even 5% of the flow of air sent to the air         separation units;     -   during the second operating phase and preferably not during the         first operating phase, liquid nitrogen and/or liquid air is sent         to the air separation units, the liquid nitrogen and/or the         liquid air being produced during the first operating phase, and         preferably not during the second operating phase, by the air         separation unit or units;     -   there are n air separation units, n preferably being at least 2,         and at least one of the air separation units or the air         separation unit has a nominal oxygen-rich gas capacity lower         than the nominal oxygen-rich gas capacity of the consumer         divided by n;     -   n is at least equal to three and at least two of the air         separation units have a nominal oxygen-rich gas capacity lower         than the nominal oxygen-rich gas capacity of the consumer         divided by n;     -   there are n air separation units, n preferably being at least 2,         and at least one of the air separation units or the air         separation unit has a nominal oxygen-rich gas capacity higher         than the nominal oxygen-rich gas capacity of the consumer         divided by n; and     -   n is at least equal to three and at least two of the air         separation units have a nominal oxygen-rich gas capacity higher         than the nominal oxygen-rich gas capacity of the consumer         divided by n.

For feasibility and/or reliability reasons, each plant generally comprises at least two air separation units. Each separation unit comprises a water/carbon dioxide purifier, for purifying the air, and also a cold box in which the distillation columns are placed. To compress the air, at least as many air compressors as there are air separation units are provided, and therefore, in the case of two separation units, there are at least two air compressors. These compressors are optionally combined with air boosters.

The plant also includes a system for storing liquid products (liquid oxygen, liquid nitrogen and possibly liquid air) consisting of one or more storage tanks per product. This storage system may be communed with these air separation units.

The air compressors and air boosters may be networked so as to supply in common all of the air separation units.

During certain, or even all, operating periods of the consumer (first operating phase), the consumer consumes a substantially constant amount of oxygen.

This constant amount is delivered permanently by the air separation unit or units. During this operating phase, the electricity cost is above a first price threshold and the oxygen consumption by the consumer is above a first consumption threshold. Throughout the first operating phase, some of the oxygen is still produced from oxygen stored and produced during the second operating phase. The oxygen coming from the storage tank may be boiled off in a reboiler external to the air separation units, but it is more energetically beneficial for the air separation units to be refrigerated via the latent heat of the oxygen coming from the storage tanks.

Liquid nitrogen and/or liquid air may be produced in periods of high electrical energy demand (tariff above the first price threshold) on the grid by supplying an element of the separation unit(s) with liquid oxygen, while the air separation units are supplied with liquid oxygen coming from a storage tank of the storage system, a storage tank of the cold box or an external source.

During periods in which the combustion unit is shut down, at least one of the air separation units continues to operate and produces large amounts of liquid oxygen, one column of the air separation unit being optionally supplied with liquid nitrogen and/or liquid air coming from a storage tank of the storage system, a storage tank of the cold box or an external source. Preferably, the number of air separation units operating while the consumer is shut down is larger than the number of air separation units operating when the consumer is in operation. In this way, the user benefits from the low electricity tariff during the second operating phase for making liquid oxygen that will serve for supplying the consumer during the first operating phase when electricity is expensive.

The invention will be described in greater detail with reference to the FIGURE, which shows a plant capable of operating according to the process of the invention.

The plant comprises a combination 1 of four air separation units, a storage system 2 and an oxygen-rich gas consumer 3, which may be a carbon fuel combustion unit or a gasifier. If it is a combustion unit, the consumer may also be supplied with air instead of oxygen.

Each air separation unit comprises a purifier 5A, 5B, 5C, 5D and a cold box 7A, 7B, 7C, 7D, the units being substantially identical.

The air separation units may receive air from four air compressors 3A, 3B, 3C, 3D connected via a common line 9 so that they can supply all the air separation units.

In a first operating phase, the oxygen-rich gas consumer 3 receives this gas from at most three of the air separation units. During this first operating phase, the electricity cost exceeds a first price threshold and is expensive. It is therefore desirable to reduce as far as possible the electricity consumption during this operating phase. For this purpose, at most three of the air separation units, or even at most two of the air separation units, are made to operate or preferably only at most three air compressors, or even at most two air compressors, are made to operate, the cold boxes operating in reduced operating mode.

Air from the two or tree operating compressors is sent to the two or three air separation units and is distilled in the columns placed in the cold boxes in order to form an oxygen-rich gas at low pressure. This pressure rarely exceeds 5 bar abs. The oxygen may be withdrawn in gaseous form from the low-pressure column of a double column. The double columns may be units having two condensers in the low-pressure column, as is known. It is also possible to boil off a liquid withdrawn from the column, taking the usual precautions for low-pressure boiling.

To compensate for the difference between the gaseous oxygen distilled in the columns and the consumption of oxygen by the consumer 3, liquid oxygen 13 is sent from the storage system 2 to the operating air separation units so that the refrigeration thereof is used intelligently in the separation units. The gaseous oxygen thus formed becomes a portion of the oxygen-rich gas 17 sent to the consumer 3. Preferably, during this first operating phase, no stream of liquid oxygen is sent from the air separation units to the storage system. Optionally, a flow of liquid oxygen not exceeding 1%, preferably 2% or even 5% of the air may be sent from the air separation units to the storage system.

During the second operating phase, the consumer 3 is not operating and therefore no flow of oxygen-rich gas is sent to this unit or the flow sent to the unit does not exceed 2% of the air sent to the air separation units. In this case, the electricity cost is below a second price threshold, the second price threshold being lower than the first price threshold, and the electricity is therefore comparatively inexpensive.

Here it is useful to operate more air separation units and/or more air compressors than during the first operating phase. Thus, if two separation units were in operation during the first operating phase, during the second operating phase three or four separation units are in operation, and if three separation units were in operation during the first operating phase, during the second operating phase four separation units are in operation. Likewise in respect of the compressors, if two compressors were in operation during the first operating phase, during the second operating phase three or four compressors are in operation, and if three compressors were in operation during the first operating phase, during the second operating phase four compressors are in operation.

In the second operating phase, the production of gaseous oxygen by the separation units becomes marginal, or even nonexistent. The production of gaseous oxygen may represent up to 1%, preferably 2% or even 5% of the supply air, this oxygen being vented to atmosphere. However, the air separation units produce all the liquid oxygen 11 which is sent to the storage system 2. The storage system 2 is filled with liquid oxygen during the second operating phase but not during the first operating phase, and is drained of liquid oxygen during the first operating phase but not during the first operating phase. However, it is possible to drain off very small amounts of liquid from the storage tank during the second operating phase.

The refrigeration of the separation unit is maintained during the second operating phase partially by sending liquid nitrogen and/or liquefied air to the air separation unit or units. This supply of liquid nitrogen and/or liquefied air does not take place during the first operating phase, and preferably liquid nitrogen and/or liquefied air is produced during the first operating phase and sent to the storage system 2. The liquid nitrogen and/or the liquid air may be sent, at least partly, to a column of the separation unit, to a separator pot or to a heat exchanger of the separation unit. 

1-15. (canceled)
 16. A process for operating a plant comprising at least two air separation units, a storage system, and an oxygen-rich gas consumer comprising a carbon fuel combustion unit or a gasification unit, the consumer being capable of generating electricity at least in a first operating phase, in which process the plant operates in a plurality of operating phases: a) in the first operating phase, during which the cost of the electricity is above a first price threshold, the consumer receives an amount of oxygen-rich gas above a first consumption threshold, coming from at least one of the air separation units, and the oxygen-rich gas consists partly of oxygen stored in the storage system, which is supplied by the air separation units during a second operating phase, and partly by oxygen produced by distillation in at least one air separation unit during the first operating phase; and b) in the second operating phase, during which the cost of the electricity is below a second price threshold, the second price threshold being below the first price threshold, the consumer consumes an amount of oxygen-rich gas below a second consumption threshold, the second threshold being below the first threshold, air is separated in at least certain of the air separation units, and an oxygen-rich liquid is sent from the separation unit or from the at least one separation unit to the storage system; and c) the number of separation units in operation during the first operating phase is fewer than the number of units that are in operation during the second operating phase.
 17. The process of claim 16, wherein the second threshold is zero.
 18. The process of claim 16, wherein the number of moles of oxygen in the oxygen-rich liquid stored throughout the duration of the second operating phase is fewer than the number of moles of oxygen sent as oxygen-rich gas to the consumer during the first operating phase.
 19. The process of claim 16, in which the oxygen-rich gas consumption during the first operating phase is substantially constant.
 20. The process of claim 16, in which the amount of air sent to at least one of the air separation units, being the unit or units in operation during the first operating phase, corresponds to an oxygen production at least 15% less than the production of gaseous oxygen sent to the consumer.
 21. The process of claim 20, wherein the amount of air sent to at least one of the air separation units, being the unit or units in operation during the first operating phase, corresponds to an oxygen production at least 25% less than the production of gaseous oxygen sent to the consumer.
 22. The process of claim 20, wherein the amount of air sent to at least one of the air separation units, being the unit or units in operation during the first operating phase, corresponds to an oxygen production at least 40% less than the production of gaseous oxygen sent to the consumer.
 23. The process as claimed in claim 20, in which the difference between the production of oxygen-rich gas corresponding to the amount of air sent to the at least one separation unit in operation during the first operating phase and the consumption of oxygen-rich gas sent to the consumer corresponds to at least part of the amount of liquid oxygen stored during the second operating phase.
 24. The process of claim 16, in which the number of separation units in operation during the first operating phase is at least two fewer than the number of units in operation during the second operating phase.
 25. The process of claim 16, in which the number of air compressors supplying an air separation unit and in operation during the first operating phase is at least two fewer than the number of air compressors supplying an air separation unit in operation during the second operating phase.
 26. The process of claim 16, in which the amount of liquid oxygen sent from the air separation unit or units to the storage system during the first operating phase does not exceed 1% of the flow of air sent to the air separation unit(s).
 27. The process of claim 26, wherein the amount of liquid oxygen sent from the air separation unit or units to the storage system during the first operating phase does not exceed 2% of the flow of air sent to the air separation unit(s).
 28. The process of claim 26, wherein the amount of liquid oxygen sent from the air separation unit or units to the storage system during the first operating phase does not exceed 5% of the flow of air sent to the air separation unit(s).
 29. The process of claim 16, in which the amount of liquid oxygen sent from the storage system to the air separation unit or units during the second operating phase does not exceed 1% of the flow of air sent to the air separation unit(s).
 30. The process of claim 29, wherein the amount of liquid oxygen sent from the storage system to the air separation unit or units during the second operating phase does not exceed 2% of the flow of air sent to the air separation unit(s).
 31. The process of claim 29, wherein the amount of liquid oxygen sent from the storage system to the air separation unit or units during the second operating phase does not exceed 5% of the flow of air sent to the air separation unit(s).
 32. The process of claim 16, in which the amount of gaseous oxygen withdrawn from the air separation units during the second operating phase does not exceed 1% of the flow of air sent to the air separation units.
 33. The process of claim 32, wherein the amount of gaseous oxygen withdrawn from the air separation units during the second operating phase does not exceed 2% of the flow of air sent to the air separation units.
 34. The process of claim 32, wherein the amount of gaseous oxygen withdrawn from the air separation units during the second operating phase does not exceed 5% of the flow of air sent to the air separation units.
 35. The process of claim 16, in which, during the second operating phase liquid nitrogen and/or liquid air is sent to the air separation units, the liquid nitrogen and/or the liquid air being produced during the first operating phase by the air separation unit or units.
 36. The process of claim 35, wherein during the second operating phase and not during the first operating phase, liquid nitrogen and/or liquid air is sent to the air separation units, the liquid nitrogen and/or the liquid air being produced during the first operating phase, and not during the second operating phase, by the air separation unit or units.
 37. The process of claim 16, in which there are n air separation units, n being at least 2, and at least one of the air separation units has a nominal oxygen-rich gas capacity lower than the nominal oxygen-rich gas capacity of the consumer divided by n.
 38. The process as claimed in claim 37, in which n is at least equal to three and at least two of the air separation units have a nominal oxygen-rich gas capacity lower than the nominal oxygen-rich gas capacity of the consumer divided by n.
 39. The process of claim 16, in which there are n air separation units, n being at least 2, and at least one of the air separation units has a nominal oxygen-rich gas capacity higher than the nominal oxygen-rich gas capacity of the consumer divided by n.
 40. The process as claimed in claim 39, in which n is at least equal to three and at least two of the air separation units have a nominal oxygen-rich gas capacity higher than the nominal oxygen-rich gas capacity of the consumer divided by n. 